CA2464392C - Spiral geometry for fuel cells and related devices - Google Patents

Abstract

A solid electrolyte fuel cell component is formed by tape casting an electrolyte layer (42) and electrode layers (40, 44) to form a green tape which can be manipulated. The green tape is coiled into a form having an S-shape central portion (50) having oppositely-directed loops, so as to provide a first longitudinal channel (46) presenting an anode surface and a second longitudinal channel (48) presenting a cathode surface. After coiling, the assembly is fired to produce a solid, sintered product.

Description

1 "Spiral Geometry for Fuel. Cells and Related Devices"

3 This invention relates to solid oxide fuel cells, and to 4 devices similar to fuel cells for use in electrocatalysis and electrolysis in gas based processes.

7 Despite considerable research and development effort, fuel 8 cells have not yet been successfully commercialised.
9 Gradual progress has been made in developing solid oxide fuel cells in two basic arrangements, flat plate and 11 tubular but costs remain high and there are sealing and 12 interconnect problems.

14 The present invention seeks to provide a radical means of addressing these problems.

17 The present invention provides, in one aspect, a method of 18 making a component having an anode, a cathode and a solid 19 electrolyte, the method comprising using tape casting to produce a green tape which is cohesive but flexible, 21 manipulating the green tape to produce a desired shape and 22 then firing the green tape to produce a rigid component;
23 the green tape comprising at least three layers each of 24 which is derived from a respective slurry comprising metal/ceramic particles dispersed in a carrier liquid; and 26 wherein the step of manipulating the green tape to produce 27 a desired shape before being fired comprises the step of 28 winding the green tape to produce oppositely directed 29 loops in the centre of the component to form longitudinal channels separated by a central web, one of the channels 31 being enclosed by an anode surface of the tape and the 32 other by a cathode surface.

1 From another aspect, the invention provides a component 2 for use in a solid oxide fuel cell, the component having a 3 generally elongate tubular form divided by a central web 4 into two channels, one of the channels being bounded by an anode surface to material flowing therethrough, and the 6 other channel being bounded by a cathode surface to 7 material flowing therethrough, the component further 8 comprising a solid electrolyte between said anode and 9 cathode; said component being formed by winding a flexible to tape having an anode layer, an electrolyte layer and a 11 cathode layer to produce oppositely directed loops in the 12 centre of the component to form said longitudinal channels 13 separated by said central web.

The invention further provides fuel cells comprising 16 components in accordance with, or made by the method of, 17 the invention.

19 Preferred features of the invention and its advantages will be apparent from the following description and claims.

22 Embodiments of the invention will now be described, by way 23 of example only, with reference to the drawings, in which:

Fig. 1 illustrates the construction and operation of 26 a known type of fuel cell;

1 Fig. 2 is a schematic side view showing an 2 apparatus used for tape casting;
3 Fig. 3 is a similar view of an apparatus used in 4 the invention;
Fig. 4 is a schematic perspective view of a fuel 6 cell component in accordance with the invention;
7 Fig. SA is a side view of a modified form of fuel 8 cell component;
9 Fig. 5B is a side view of the modified component following a first step to produce a seal at one end;
11 and 12 Figs. 5C and 5D are side and plan views, 13 respectively, of the component following a second 14 step.

16 Background 18 Referring to Fig. 1, a solid oxide fuel cell 19 comprises an anode 10, a cathode 12, and a solid electrolyte 14. The cell produces electricity by 21 electrochemically combining hydrogen (which may be 22 present as such, or in a hydrocarbon fuel) and 23 oxygen (which may be present as such or in air).
24 The oxygen is reduced at the cathode 12, accepting electrons from the external circuit to form 02- ions 26 (equation (1)) which are conducted through the solid 27 electrolyte 14 to the anode 10. At the 28 anode/electrolyte interface, hydrogen is oxidised to 29 form H20, releasing electrons back into the external circuit (equation(2)).

32 02 + 4e- 202- (1) 1 2H2 + 202- -- 2H20 + 4e- (2) 3 Each of the three components must not react with any 4 other component it is in contact with, must be stable at operating temperatures, and all three must 6 have similar thermal expansions. The anode 10 and 7 cathode 12 need high electronic conductivity and 8 sufficient porosity to allow the gases to reach the 9 electrode/electrolyte interface. In comparison, the electrolyte must be dense, preventing gas flow, have 11 high oxygen ion conductivity, allowing 02- ions to 12 permeate with minimum resistance, and as small an 13 electron transport number as possible.

One known family of fuel cells uses yttria 16 stabilised zirconia (YSZ). The anode consists of 17 YSZ mixed with Ni, and the cathode of YSZ mixed with 18 Sr doped LaMnO3. This serves to obtain similar 19 thermal expansion to the electrolyte, and also acts to increase the triple phase boundary (the area of 21 contact between anodic/cathodic material, 22 electrolytic material, and the gas phase).

24 Two main types of fuel cell exist at present. One is the planar cell, in which flat plates in the 26 geometry shown in Fig. 1 are stacked one on top of 27 another separated by an interconnect. The other is 28 tubular, in which the materials are formed into 29 tubes with the inside surface cathode and the outer surface anode. Air and fuel (hydrogen source) are 31 passed over the corresponding electrodes.

1 Preferred Embodiments 3 Turning to Fig. 2, the present invention makes use 4 of a process of tape casting to form the electrode 5 and electrolyte structures. Tape casting as a 6 process is known per se, see for example `Tape 7 Casting Theory and Practice' by Richard E Mistler 8 and Eric R Twiname, but has previously been used in 9 the field of fuel cells only to manufacture single layers such as anodes or cathodes.

12 Tape casting is the production of thin sheets of 13 ceramic and/or metallic material. The 14 ceramic/metallic powders are mixed by ball mill together with various organic materials: solvent, 16 dispersing agent, binder and plasticizer which hold 17 the individual particles in a homogeneous 18 distribution throughout the slurry.

As seen in Fig. 2, the slurry 20 is cast onto a 21 moving carrier surface 22 by a doctor blade 24. The 22 carrier surface 22 may suitably be a glass plate or 23 Mylar sheet. Upon evaporation of the solvent, a 24 flexible `green' tape is produced which may be handled and manipulated. The green tape is 26 subsequently fired, removing the remaining organic 27 material and producing a hard, rigid sintered 28 material.

The ball milling stage is important to ensure that 31 all the soft agglomerates are broken down and the 32 powder is well dispersed. The ball milling is 1 normally performed on the powder, solvent and 2 dispersant; the binder and plasticizer added 3 subsequently, and the entire mix may undergo further 4 ball milling but at a slower speed. De-airing the slurry and maintaining a constant casting speed 6 ensure constant thickness and smooth surface finish 7 of the green tapes.

9 Fig. 3 shows an apparatus in which three slurries 20a, 20b, 20c are cast sequentially on a single 11 carrier surface 22, thus producing a three-layer 12 green tape which can be handled as a single unit and 13 fired to produce a rigid unitary structure. By 14 using suitable materials in the three slurries, a fuel cell component comprising anode, cathode and 16 solid electrolyte is produced. A preferred 17 composition is:
18 anode YSZ and NiO which is reduced to 19 Ni under fuel conditions cathode YSZ and Sr doped LaMnO3 21 electrolyte YSZ (8-10 mol% yttria, balance 22 zirconia) 23 One alternative to the multiple casting arrangement 24 of Fig. 3 is as follows. The electrolyte layer is deposited first, and one electrode layer is 26 deposited on top, once the electrolyte layer has 27 partially dried. This composite is allowed to dry 28 somewhat, after which the two-layer composite is 29 turned over and the second electrode layer deposited on top.

1 Another alternative is to produce three separate 2 ribbons by tape casting, and combine these by 3 stacking and applying pressure, for example by 4 passing between rollers. This has the advantage of further reducing the electrolyte thickness.

7 The three layer structure produced by any of the 8 foregoing methods forms a single component which can 9 be handled and fired as a unit (co-fired). This contrasts with prior art use of tape casting, where 11 each electrolyte or electrode layer is formed and 12 fired separately.

14 These fuel cell components can be produced simply by tape casting and firing, resulting in flat plate 16 components. However, the invention also provides a 17 novel form of fuel cell which is made possible by 18 the use of tape casting.

Referring to Fig. 4, a three layer tape having anode 21 40, electrolyte 42 and cathode 44 is wound while in 22 the green state prior to firing. The winding is 23 such as to produce oppositely-directed loops in an 24 S-shape in the centre of the component, thus forming longitudinal channels 46 and 48 separated by a 26 central web 50. One channel 46 has a surface of 27 anode material 40,E while the other channel 48 has a 28 surface of cathode material 44. Typically, the 29 overall cross-section of the wound component may be about 50 mm, and the channels 46 and 48 each have a 31 width of about 5 mm. The component may be wound 32 from a tape 0.2m x 2m.

2 In use, air is passed through the channel 48 to 3 contact the cathode 44, and hydrogen (or a hydrogen-4 containing fuel) is passed through the channel 46 to contact the anode 40. The anode and cathode are 6 porous, preferably about 50% porosity, and thus the 7 air and hydrogen permeate through the anode and 8 cathode layers and are not simply in contact with 9 the parts fronting the channels 46 and 48.

11 The arrangement shown in Fig. 4 thus provides a fuel 12 cell component which is simple to make, gives a 13 large active area within compact dimensions, and 14 combines the best features of flat plate and tubular fuel cell geometries.

17 Fig. 5 illustrates a modification of the embodiment 18 of Fig. 4. This makes use of the fact that the 19 electrolyte layer 42 is dense and impermeable. In Fig. 5, the electrolyte layer 42 is of greater width 21 than the electrode layers 40 and 44 and thus forms 22 projecting portions 42a, 42b when the layers are 23 wound or coiled. The projecting portion 42a is 24 pressed (Fig. 5A) to form a flattened end (Fig. 5B) which is then turned over (Figs. 5C and 5D) to form 26 a seal, in the manner of a toothpaste tube. The 27 assembly is then fired to form a rigid component 28 sealed at one end.

The projecting portion 42b at the other end may be 31 used for connecting the component to a gas supplies 32 such as fuel and air manifolds.

2 Choice of materials 4 The foregoing embodiment is based upon the use of YSZ materials. Such materials are presently 6 preferred in carrying out the invention, and it is 7 believed that the use of high-zirconia materials 8 will be of particular benefit when using co-firing 9 of multiple tape layers. However, other materials may be used in implementing the invention.

12 The electrolyte should be an ionically conducting 13 oxide capable of transporting either oxygen ions or 14 protons or both. Typical materials in addition to yttria-zirconia are scandia-stabilised zirconia, 16 cerium oxide based materials, lanthanum gallate 17 materials, and oxide proton conductors such as 18 barium cerate, strontium zirconate, and other 19 perovskites based on cerium, niobium or zirconium, and titanium containing alkaline earth strontium or 21 barium or rare earths or yttrium or scandium.

23 Alternative air electrode materials would be based 24 on lanthanum strontium cobaltate, lanthanum strontium iron oxide, and various combinations of 26 manganese cobalt and iron in the same perovskite 27 lattice.

29 The fuel electrode in addition to nickel zirconia cermets may use copper zirconia cermets, copper 31 ceria cermets, nickel ceria cermets, perovskites 32 based on lanthanum chromate, and fluorites based on 1 yttria zirconia titania either on their own or in 2 combination with a current collecting material.

4 In summary, the invention may be applied to any 5 oxide fuel cell having an electrolyte with solely 6 oxide or/and proton ionic activity and electrodes 7 with appropriate catalytic, electronic and ionic 8 activity to function in the reduction of air (or 9 oxygen or other oxidant) and the oxidation of 10 hydrogen, hydrocarbon, reformed hydrocarbon or other 11 appropriate fuel.

13 Process Examples Some specific examples of tape casting YSZ-based 16 slurries and tape processing will now be given.

18 Two sources of YSZ powder have been used. A first 19 powder was obtained from Pi-Kem Ltd and has the following analysis:

24 wt%
Y203 13.62 26 Si02 0.01 27 TiO2 0.002 28 Fe203 0.003 29 CaO 0.002 A1203 0.25 31 Na20 0.003 32 L 0 1 0.07 2 Balance: Zirconia 4 Average particle size: 0.21 m Surface area: 6.9 m2/g 7 The other powder was by Tioxide Ltd; no analysis is 8 available. The powder by Tioxide Ltd was premixed 9 with a binder, but the binder was removed by heating at 600 C overnight.

12 Particle size distribution was measured, without de-13 flocculation, by an LS Particle Size Analyser with 14 detection limits of 0.4 m to 2000 m. 10 second ultrasonic agitation was performed prior to 16 detection. The largest particles detected were 4 m 17 (Pi-Kem Ltd) and 5 m (Tioxide Ltd) and both powders 18 contained particles smaller than 0.4 m. The LS
19 Particle Size Analyser showed the mode particle size to be 1.43 m (Pi-Kem Ltd) and 1.72 m (Tioxide 21 Ltd).

23 A number of dispersing agents were investigated, 24 namely tri-ethanol amine, citric acid, menhaden fish oil, oleic acid, phosphate ester (acid form), and 26 polyethylene glycol. Tri-ethanol amine was found to 27 work well with the Tioxide Ltd product, and 28 phosphate ester (acid form) with the Pi-Kem Ltd 29 product provided the quantity was kept below 1.5, preferably 0.05 - 0.12, g per 10 g of YSZ.

1 Tapes were produced using a planetary ball mill and 2 YSZ by Tioxide Ltd, with polymethyl methacrylate 3 (PMMA) and polyvinyl butyral (PVB) as binders. The 4 slurry compositions were as follows:

a) Binder: PMMA

Chemical Mass/g(2 dp) Powder YSZ (Tioxide Ltd) 10.00 Solvent Methyl ethyl ketone/ethanol 5.20 (6:4 wt%) Dispersant Tri-ethanol amine 0.25 Binder PMMA 2.24 Plasticizers Polyethylene glycol (MW300) 1.62 Di-butyl phthalate 1.46 b) Binder: PVB

Powder YSZ (Tioxide Ltd) 10.00 Solvent Methyl ethyl ketone/ethanol 5.20 (6:4 wt%) Dispersant Tri-ethanol amine 0.24 Binder PVB 1.12 Plasticizers Polyethylene glycol (MW300) 0.81 Di-butyl phthalate 0.73 8 The tapes produced were flexible, with less binder 9 required when using PVB, showing PVB to have better binding properties. For both tapes, ease of removal 11 was better from a glass carrier than from a Mylar 12 carrier.

2 Tapes with PVB binder were noted to be `sticky' and 3 if coming into contact with themselves were 4 difficult to prise apart. TGA analysis showed both binders were completely removed by 600 C.

7 The tapes were cut into sections and subjected to 8 various firing rates and temperatures. They were 9 fired flat, onto a Safil firing block.

11 Slow heating of 1.5 C/min to 600 C,removing the 12 organic-material, greatly increased tape porosity.
13 The PMMA binder tape has a larger pore size than the 14 PVB binder tape, due to the higher binder: powder radio. Both tapes were very brittle.

17 Slow heating of 1.5 C/min to 600 C, rapidly heating 18 to 1000 C (11.5 C/min) and holding at this 19 temperature for 5 hours, again showed the tapes produced with PMMA binder to be more porous.

21 Comparison to the tapes heated to 600 C show a 22 decrease in porosity after the temperature increase 23 as the tapes contracted. The tapes were less 24 brittle after firing at 1000 C, but were still easily broken.

27 Tapes were subjected to rapid heating of 11.5 C/min 28 to 1000 C and holding at this temperature for 5 29 hours. The tapes are still porous, but interestingly, there is an obvious decrease in 31 porosity for tapes from PMMA binder and an increase 32 in porosity for tapes from PVB binder without the 1 slow binder removal stage. Again, these tapes were 2 brittle.

4 Sintering at 1500 C for 5 hours after slow binder removal reduced porosity further. The thickness was 6 124 m (PVB binder) and the porosity of the PMMA
7 tape to be much higher - reflected by the greater 8 strength of the PVB binder tape. Both tapes 9 sintered well. Impurities and many holes were present on both tapes. Impurities could be due to 11 dust particles, or Si particles picked up from the 12 furnace block.

14 A small sample of green tape was rolled according to the geometry in Figure 4, and fired to 1500 C.

16 Although the above flat tapes showed a smooth 17 surface finish, the rolled tapes did not. This was 18 thought to be due to too fast a heating rate causing 19 the organic material to bubble leaving bumps on the surface.

22 Intense mixing of the planetary ball mill is thought 23 to have adverse effects on the binder and further 24 tapes were produced using PVB binder for YSZ

obtained from both Pi-Kem Ltd and Tioxide Ltd, with 26 the rotary ball mill.

28 Green tapes produced with YSZ (Tioxide Ltd) by 29 rotary and planetary ball mill were compared. Both ball mills produced a similar homogenous particle 31 distribution, although more `lumps' are seen in the 32 planetary ball milled tape. This is possibly due to 1 the more effective mixing of the planetary ball mill 2 meaning the slurry was mixed for too long. Mixing 3 of the slurry after binder addition for too long has 4 the effect of producing less dense tapes, due to the 5 substitution of the dispersant by the binder causing 6 the `zipper bag' effect, where the binder wraps 7 around a group of particles to form an agglomerate.

9 The tapes were heated at 0.8 C/min to 600 C, then to 10 1000 C at 1.5 C/min, followed by 3.5 C/min to 1500 C
11 and sintered at 1500 C. The thickness of the tape 12 sintered at 1500 C was found to be much less than 13 the planetary ball milled sample at 82 m. Halving 14 doctor blade gap height gave a decreased thickness 15 to 45 m. Both tapes show a decrease in porosity 16 when produced with the rotary ball mill.

18 Again, the tapes sintered well. However, localised 19 holes were still present and impurities were seen in grain boundaries.

22 YSZ powder from Pi-Kem Ltd was milled in a rotary 23 ball mill. The slurry composition was as follows:-Chemical Mass/g(2dp) Powder YSZ (Pi-Kem Ltd) 20.00 Solvent Methyl ethyl 10.45 ketone/ethanol (6.4 wt%) Dispersant Phosphate Ester (acid 0.21 form) Binder PVB 2.24 Plasticizers Polyethylene glycol 1.62 (MW300) Di-butyl phthalate 1.46 2 The green tape shows a higher porosity than the 3 green tape produced from YSZ (Tioxide Ltd) 4 particles. However, the relative viscosity of the two slurries, suggests that the YSZ (Pi-Kem Ltd) 6 particles were much better dispersed.

8 The tape was shaped into the desired geometry 9 (Figure 4). They were heated to 600 C at 0.5 C/min, then to 1000 C at 0.8 C/min, followed by heating to 11 1500 C at 10 C/min and sintering at 1500 C for 5 12 hours. In order to reduce the impurities, an 13 alumina plate was placed between the firing block 14 and the samples. Tape thickness was greater than the tapes produced by YSZ (Tioxide Ltd)at 76 m, and 16 the tape was denser. Increase in thickness and 17 density could be explained by decrease in slurry 18 viscosity.

The main surface showed fewer impurities, but 21 contained more holes. This could be attributed to 22 the geometry effectively increasing tape thickness, 23 hence more organic material having to pass through 24 the outer surface.

26 It was found that towards the centre of the sintered 27 rolled tape the layers of tape are in contact with 28 each other and sintered together. However, the 1 outer layer is only in contact with the rest of the 2 sample in small sections.

4 PVB was shown to be a more effective binder than PMMA for production of green tapes. The smaller 6 quantities of PVB required with respect to PMMA lead 7 to denser tapes.

9 The time-scale used for ball milling (recommended by `Tape Casting Theory & Practise' by Richard E

11 Mistier and Eric R Twiname) shows use of the 12 planetary ball mill produces more porous films.

14 The increased number of holes in the rolled tape's surface may be reduced when fired with porous anode 16 and cathode, providing an easier escape route for 17 the organic material.

19 Further Examples 21 The following examples of slurry formulations have 22 been found to be better optimised than those 23 presented above.

Electrolyte formulations 26 YSZ 30.00g 27 Solvent 14.50g MEK:ethanol 6:4 by weight 28 Dispersant 0.195g Triton 0.44 29 Binder 3.36g PVB
Plasticisers 2.43g polyethyleneglycol 31 2.19g di-butylphthalate 1 Procedure 2 1. 14g solvent + powder + dispersant. Ball mill 3 18hrs at about 160rpm.
4 2. Add plasticisers + binder + 0.5g solvent. Mix by vibratory mixer for about 20min. Ball mill for 6 4hrs at about 100rpm.
7 3. De-air by rolling with no milling media at about 8 6rpm for about 23hrs.
9 4. Cast on tapecaster TT-1000 from Mistier & Co.
Speed: 50%
11 Doctor blade height: 0.3048mm (0.012inch) 12 Carrier: Mylar 14 Anode formulations YSZ 5.8633g) 16 NiO 7.2570g) weighed correct (by balance) 17 Graphite 4.0984g) to +/-0.0002g 18 Solvent 10.125g MEK:ethanol 6:4 by weight YSZ:NiO equivalent to 60:40 of YSZ:Ni by volume on 21 reduction 23 NiO+YSZ:graphite is 50:50 by volume Procedure 26 1. Ball mill for 18 hours at 160rpm (ball mill has 27 both rocking and rolling action) with 28 Binder PVB 2.52g 29 Plasticiser di-butylphthalate 1.643g PEG 1.823g 31 Note: no dispersion agent added 1 2. De-air. Ultrasonic agitation 30min. Vacuum 2 SinchHg (below atmospheric) 5min.

4 Modifications 6 The above description refers to electrodes each 7 consisting of a single uniform layer of sintered 8 material. However, each of the electrodes could be 9 constituted by composite layers which together fulfil the functions of the electrode, namely 11 catalytic performance, electrochemical performance, 12 electronic conduction, and gas distribution.

14 The anode and cathode may each be formed by two or more tapes laminated together to provide a gradation 16 of function. Also, meshes or ribbons may be 17 interspersed between the plural tapes, the meshes or 18 tapes being burnt out during firing to form gas 19 distribution channels. Alternatively the tapes may be appropriately scored using a serrated doctor-21 blade to provide such channels. In one example of 22 cathode, a porous layer is formed next to the 23 electrolyte from a mixture of YSZ and lanthanum 24 strontium manganite or other electrode material, and a current collection layer with built-in channels is 26 deposited on top of this, made from lanthanum 27 strontium manganate.

29 An alternative material to nickel may be used to bridge the gap between the high temperature of the 31 fuel cell anode and the low temperature of the 32 incoming gas stream, suitably materials based on 1 oxides such as lanthanum chromite. Indeed, the 2 anode itself, or part of the anode, may be formed 3 from oxide materials such as lanthanum chromite.

7 Summary 9 It will be appreciated that the process examples 10 given above are by way of explanation of general 11 principles, rather than precise examples of specific 12 formulations. However, from this information the 13 person skilled in the art will be able to arrive at 14 suitable compositions and processes for practising 15 the invention, with no more than routine 16 experimentation.

18 Although the preferred form of the invention is the 19 S-shaped looped coil as shown in Fig. 4, the 20 invention also includes the production of flat plate 21 fuel cell components by firing flat tapes.

22 Moreover, by simple rolling up of tapes followed by 23 firing, tubular fuel cell components may be 24 produced.
26 Materials other than YSZ may be used, for example 27 scandia stabilised zirconia or scandia + yttria 28 stabilised zirconia, suitably 8 -14 mol% scandia +
29 yttria, remainder zirconia; and other materials as discussed above.

1 Although described with particular reference to fuel 2 cells, the invention may also be applied to devices 3 for use in electrocatalysis or electrolysis in a 4 range of gas based processes.

6 Other modifications and improvements may be made to 7 the foregoing embodiments within the scope of the 8 invention as defined in the claims.

Claims (14)

1. A method of making a component having an anode, a cathode and a solid electrolyte, the method comprising using tape casting to produce a green tape which is cohesive but flexible, manipulating the green tape to produce a desired shape and then firing the green tape to produce a rigid component; the green tape comprising at least three layers each of which is derived from a respective slurry comprising metal/ceramic particles dispersed in a carrier liquid; and wherein the step of manipulating the green tape to produce a desired shape before being fired comprises the step of winding the green tape to produce oppositely directed loops in the centre of the component to form longitudinal channels separated by a central web, one of the channels being enclosed by an anode surface of the tape and the other by a cathode surface.

2. A method according to claim 1, in which the component is a solid oxide fuel cell component.

3. A method according to claim 1 or claim 2, in which the green tape is formed by casting at least three slurries one on top of the other and allowing the carrier liquid to evaporate.

4. A method according to claim 1 or claim 2, in which the green tape is formed by casting at least three separate ribbons and pressing these together.

5. A method according to claim 4, wherein the step of pressing the ribbons together comprises passing the ribbons through rollers.

6. A method according to any one of claims 1 to 5, in which one or both of the anode and the cathode is formed by plural layers cast from slurries of differing composition.

7. A method according to claim 6, in which there is interposed between said plural layers a web or mesh of a material which burns away during firing to leave gas flow passages in the formed electrode.

8. A method according to any one of claims 1 to 7, in which the carrier liquid comprises a solvent optionally combined with one or more of a dispersant, a binder, and a plasticiser.

9. A method according to any one of claims 1 to 8, in which the particles in each of the slurries are based on yttria stabilised zirconia (YSZ).

10. A method according to claim 8, in which the anode slurry comprises particles of YSZ and particles of Ni or NiO, and the cathode slurry comprises particles of YSZ and particles of Sr doped LaMnO3.

11. A method according to any one of claims 1 to 10, in which the green tape is formed with an electrolyte layer wider than the electrode layers and protruding from one side thereof, and in which, before firing, the green tape is wound into a cylindrical form and the protruding electrolyte layer is closed upon itself to form a seal at one end of the component.

12. A component for use in a solid oxide fuel cell, the component having a generally elongate tubular form divided by a central web into two channels, one of the channels being bounded by an anode surface to material flowing therethrough, and the other channel being bounded by a cathode surface to material flowing therethrough, the component further comprising a solid electrolyte between said anode and cathode; said component being formed by winding a flexible tape having an anode layer, an electrolyte layer and a cathode layer to produce oppositely directed loops in the centre of the component to form said longitudinal channels separated by said central web.

13. A component according to claim 12, in which the flexible tape is a green tape formed by slurry casting and solvent evaporation; and after being wound the component is fired to produce a rigid component.

14. A fuel cell comprising a number of components as claimed in claim 12 or claim 13.